Continuing research of this project is concerned with development of methods which can be used for the synthesis and semi-synthesis of peptides under mild conditions, particularly in aqueous solutions within "physiological" ranges of pH and temperature. Methods are being developed to specifically and reversibly protect amino groups, carboxyl groups, and several amino acid side-chain functional groups in preformed peptide fragments in aqueous solution. Specific methods for removal of such blocking groups in aqueous solutions include both mild chemical treatments and enzyme-catalyzed hydrolyses. Bifunctional reagents are being developed which can be attached to polymers or other supporting media through one functional group of the molecule. The second functional group of the reagent is a specific ligand which can form a reversible covalent linkage to the side-chain of a particular amino acid residue in aqueous solutions.